1. Field of the Invention
The present invention relates to an overcurrent protection circuit for a switching power supply which is to be installed in e.g. a laser printer, and has multiple voltage supply sections.
2. Description of the Related Art
Some of known overcurrent protection circuits for switching power supplies are of multiple output type to operate in forward mode. (Refer to, e.g., Japanese Laid-open Patent Publication 2000-322133, paragraphs 0007 to 0013 and 0022, FIG. 1.) This Patent Publication discloses a switching power supply circuit of multiple output type, in which a transformer to perform the switching operation comprises, for output at secondary side thereof, a lower voltage supply section of 3.3 V and a higher voltage supply section of 5 V. In this switching power supply circuit, an electric current value flowing through a resistor provided in an output line at secondary side of each voltage supply section is detected by a comparator for overcurrent detection (for overcurrent protection). When it is detected by the comparator that the electric current reaches an overcurrent detection value, a photocoupler is turned on by the detected output, thereby controlling on-duty of a power switching element at primary side of the transformer so as to shut down the entire circuit.
A switching power supply circuit having such multiple output voltage supply sections has the following undesirable possibility. For example, when the output voltage of the 3.3 V voltage supply section is maintained within a rated range while the output voltage of the 5 V voltage supply section decreases, then the 5 V voltage supply section gets into an overload mode, and continues to further decrease gradually. When the output voltage of the 5 V voltage supply section decreases below approximately 3.3 V, resulting in reversal in magnitude between the two output voltages, there is a possibility that electronic components, such as ICs, providing the two voltage supply sections may be destroyed due, e.g., to latch-up phenomena. For solving such problem, according to the above-referred Japanese Laid-open Patent Publication 2000-322133, the entire power supply is shut down when the voltage of the higher voltage supply section decreases to approximately the voltage of the lower voltage supply section.
On the other hand, in a switching power supply circuit having a single, rather than multiple, output voltage supply section, it is known to provide an overcurrent protection circuit for protecting the switching power supply from an overcurrent exceeding the control limit of a primary side control circuit. (Refer to, e.g., Japanese Laid-open Patent Publication Hei 5-211715, paragraph 0012, FIG. 1). According to this overcurrent protection circuit, a decrease of the output voltage is detected, the decrease being caused when the secondary side of a transformer is brought to short-circuit state or nearly short-circuit state. Such detected signal is input, via a photocoupler, to the primary side control circuit which performs pulse-width modulation for controlling a switching element, thereby attempting to maintain the output voltage of the switching power supply at a constant value. At this time, the on-time of the switching element increases to cause an overcurrent to flow. The voltage generated by the overcurrent flow between both ends of an overcurrent detection resistor provided in the primary side of the transformer is divided and applied to the primary side control circuit in order to perform overcurrent protection by narrowing the pulse width.
Furthermore, in a switching power supply in which an overvoltage protection signal and a feedback signal from an output voltage of secondary side of a transformer are superimposed on each other and transmitted to primary side of the transformer using one photocoupler, it is known to provide an arrangement for making it possible to stop the operation of the power supply when an abnormal condition such as photocoupler failure occurs. (Refer to, e.g., Japanese Laid-open Patent Publication 2000-156972, paragraph 0021, FIG. 1.). In this power supply, the primary side of the transformer is provided with detection means for detecting the feedback signal transmitted via the photocoupler, with the feedback signal being divided into plural levels for detection.
Among the conventional technologies as described above, the switching power supply circuit having plural output voltage supply sections according to the above-referred Japanese Laid-open Patent Publication 2000-322133 has drawbacks as follows. Although the entire power supply can be shut down when the voltage of the higher voltage supply section decreases to approximately the voltage of the lower voltage supply section, a total overcurrent in the primary side of the transformer is not detected for shutting down the entire power supply. Accordingly, it is needed to provide an overcurrent detection circuit in each voltage supply section of the secondary side of the transformer. In addition, a comparator is used for the circuit, thereby causing the cost of the circuit to increase. It is to be noted that the Japanese Laid-open Patent Publication 2000-322133 does not disclose any concept that detection of an overcurrent in a low voltage supply section in a circuit having multiple output voltage supply sections may enable a countermeasure against overload in windings of a transformer for preventing e.g. insulation breakdown.
On the other hand, the overcurrent protection circuit disclosed in the Japanese Laid-open Patent Publication Hei 5-211715 does not have multiple output voltage supply sections, and does not detect an overcurrent at the secondary side of the transformer for protection operation, although the output voltage of the secondary side of the transformer is detected therein.
Likewise, the switching power supply disclosed in the Japanese Laid-open Patent Publication 2000-156972 does not detect an overcurrent at the secondary side of the transformer for protection operation, either.